This invention relates to a method for mounting an electronic part on a circuit board, and more particularly to a surface mounting method for simply and easily mounting on a circuit board an electronic part whose contacts are arranged substantially vertically with very small intervals, for example, a pin grid array type (PGA type) electronic part, particularly having a great number of contacts, and further relates to a connector use in the method.
In the hitherto used method for mounting an electronic part onto a circuit board, the latter is formed with through-apertures, terminal legs of an electronic part are inserted into the through-apertures and soldered to the circuit board.
Recently, however, a method for directly soldering terminal legs of an electronic part to terminal legs of a circuit on a circuit board has been widely used without forming apertures in the circuit board for mounting electronic parts, for example, semiconductor chips, resisters, condensers, coils, socket connectors and the like onto circuit boards. This method is the so-called surface mounting method (SMT).
In the surface mounting method, in order to make it easy to inspect the connected state between the terminal legs of the circuit board and electronic part, the terminal legs of the electronic part are made in the form of a "gull wing", J-shaped lead or the like to locate the legs out of the electronic part or at least near thereto so as to effect visible inspection by observation.
On the other hand, however, with an electronic part having a great many terminals as in the PGA socket, the length of the outer periphery of the socket is short in comparison with the number of the terminals and spaces between the terminals which are very narrow. Therefore, the terminal legs are required to be elongated from the center of the socket to the outside of the outer periphery of the socket in order to carry out the surface mounting method for the electronic part having terminal legs in the form of a "gull wing" or J-shaped lead. Consequently, the arrangement of lead lines becomes unavoidably very complicated.
The PGA socket has an inherent advantage in that distribution of lead lines in high density can be avoided by uniformly arranging the terminals. However, when the terminal legs of the PGA socket extend to the outer periphery of the socket, such an advantage is no longer accomplished. Moreover, if a circuit board is slightly waved, bent or curved and a PGA socket itself is deformed, the socket terminal legs are not tolerant of irregularity of distances from the circuit board terminals, with the result that highly reliable connections between terminals can not be ensured.
In recent years, with development of the high density mounting technique for integrated circuits, the numbers of input and output terminals per unit area of circuit boards have rapidly increased. As described above, the pin grid array system (PGA system) has been employed for mounting electronic parts such as modules of central processors for large type computers with a rapid increase in the numbers of input and output terminals.
FIGS. 1a and 1b illustrate two cases of a PGA module mounted on a circuit board without extending terminal legs from the PGA module or center of its socket to its outside according to the surface mounting method in a conventional manner.
In FIG. 1a, the PGA module 2 is directly mounted on the circuit board 1. The PGA module 2 has pin contacts 3 which are fixed to "lands" 4 formed on the circuit board 1 by means of solder 5.
In FIG. 1b, onto the circuit board 1 is first mounted a connector 6 onto which the PGA module 2 is mounted. The PGA module 2 has pin contacts 3, and the circuit board 1 has "lands" 4 of a circuit pattern thereon. The connector 6 has a base 7 which is generally made of an insulating material such as ceramic or plastic material. Contact portions 8 of contacts are provided in the base 7 of the connector 6 and adapted to be fitted with the pin contacts 3 of the PGA module 2. Terminals 9 of the contacts are mounted on the base 7 of the connector 6 and fixed to the "lands" 4 of the circuit board 1 by means of solder 5.
The two methods shown in FIGS. 1a and 1b involve the following problems to be solved.
(1) With an electronic part such as a recent PGA module having a great number of pin contacts, for example, 50 to 300 contacts arranged in rows and columns, soldered conditions of the contacts near the center could not be visibly inspected although the contacts near the outer peripheries of the module and a circuit board are easily inspected. It is very difficult to perform reliable inspection on the contacts near the center even utilizing a mirror or X-ray because of hindrance of the PGA module in both the cases shown in FIGS. 1a and 1b. PA1 (2) In the case of the FIG. 1a wherein the pin contacts 3 of the PGA module are directly soldered to the "lands" 4 of the circuit pattern on the circuit board 1, high accuracy is imposed on the lengths of the pin contacts of the PGA module. If such a requirement is not fulfilled, reliable soldering is not realized. PA1 (a) The connections of the lead pins mounted on the circuit board can be visibly observed for inspection. PA1 (b) The arrangement of the PGA lead pins can be determined utilizing all the surface of the circuit board facing to the PGA at will, and sufficient spaces between the connection terminals can be used in comparison with the case using only a periphery portion. PA1 (c) Lead lines from the terminals at the center of the connection surface of the PGA to the periphery are not needed so that a complicated lead line construction due to increase in lead lines can be avoided. PA1 (a) The circuit board pins mounted on the circuit board are completely exposed so that the heat produced in soldering the circuit board pins is uniformly distributed to ensure uniform soldering on all the circuit board pins. PA1 (b) If it is required to remove the PGA from the circuit board after mounting, the PGA can be readily removed or replaced with other one. Therefore, it is very advantageous about its preservation. PA1 (a) The high reliability is maintained, which has been already acknowledged on general connectors. In other words, even if the number of terminals of a PGA is several thousand, reliable electric connections are achieved. PA1 (b) Only the insertion into the connector immediately establishes the connection of the circuit board and the PGA.